domination of the centre!!!
domination of the centre with the queens gambit
look at QGA the games past move 4 and u will see they arent too bad for black and the central domination comes under hefty pressure in the middlegame since black can unravel easily..
After 3. Qa4 best for black would be 3...Nd7, which would then allow him to start operations in contesting the center with c5 (or perhaps even e5 if allowed.)
The Q+ to retrieve the pawn is slow in comparison to using minor pieces (Nf3, e3, Bxc4.) The pawn doesn't need to be recaptured right away since its well known that black can't hold it without giving white good advantage.